Field of the Invention
The invention concerns in general the technology of coating units that are used as integrated parts of a manufacturing line. Especially, the invention concerns the optimized operation of a coating unit that follows a sheet-fed digital printer.
Description of Related Art
Many manufacturing processes involve handling workpieces initially in planar, sheet-like form. As an example, the manufacturing process of packages is considered. The manufacturing process is typically arranged so that it takes advantage of the relatively easy handling of workpieces at the stage when they are still in planar form. A typical process for manufacturing cardboard packages comprises at least a printer, a stacker, and a die cutter in this order. Coaters, dryers, and/or other arrangements may follow the printer for implementing steps that from the viewpoint of printing represent post-processing. As an example, a coater may be disposed directly after the printer and used to apply a layer of water- or solvent-based varnish over at least parts of the printed surface.
At the time of writing this description, the printer is more and more often a sheet-fed digital printer, capable of flexibly producing short series and making fast changes to at least parts of the printed pattern(s) even after each work-piece. Compared to the relatively long and regular runs made with traditional web-fed printing presses, print works executed with a sheet-fed digital printer are frequently characterized by irregular output, meaning that pauses of variable duration may occur between consecutive workpieces and series of workpieces that come out of the printer. A consequence of the flexibility of the printer is a requirement for also the subsequent machinery to adapt their operation to the irregularities in operation.
As an example, consider a flexographic coating unit like the one schematically illustrated in FIG. 1. Printed sheets come from the left in the drawing, pass between a plate cylinder 101 and an impression cylinder 102, and continue to the right in the drawing to be stacked and/or transported further to die-cutting. An inking arrangement, shown schematically to comprise a fountain roller 103 and an anilox roller 104 in FIG. 1, is used to dose varnish or some other coating substance onto the surface of the plate cylinder 101. Some kind of transport arrangement is needed in order to keep the workpieces moving, because unlike the material web in web-fed processes, the sequence of separate sheet-like workpieces cannot be drawn from ends. In FIG. 1, vacuum belts 105 have been illustrated as an example of a transport arrangement.
If the coating substance is to be applied in specific patterns, the mirror images of corresponding patterns have been formed in positive (as elevated areas) on the surface of the plate cylinder. The coating substance then only becomes spread on the elevated areas, and consequently forms the desired patterns on the printed surface when the surface of the plate cylinder presses against the appropriate workpiece. The “printing plate”, as the outmost surface layer of the plate cylinder is called, is made of flexible material such as a selectively hard-ened light-sensitive polymer, which explains the descriptor “flexographic”.
A particular disadvantage of prior art coating units was their tendency of becoming contaminated or even clogged with leftover coating substance. Not only the outer surface of the plate cylinder, but also parts of the machinery where no coating substance should appear in the first place, slowly but certainly accumulate contamination that originates, e.g., from unintended splashes and small amounts of coating substance spreading around in aerosol form. This disadvantage becomes more prominent with water-based varnish than with UV-hardened coating substances.
Prior art is known from Japanese Patent Application JP2004181899A which discloses a coater printing plate varnish drying prevention device, U.S. Pat. No. 2,894,481 which discloses control devices for apparatus for applying coatings to metal sheets, German Patent Application DE19523879A1 which discloses a sheet conveying system in a digital printing press and from U.S. Pat. No. 8,251,498 B2 which discloses a processing liquid applying apparatus and image-forming apparatus.